Method for straight-through cracking of hydrocarbons

ABSTRACT

HYDROCARBON FEED STOCKS ARE CRACKED BY PASSING THEM IN A STRAIGHT LINEAR PATH THROUGH ELONGATED EXTERNALLY HEATED TUBES OF OPEN CROSS-SECTION, WITH A RESIDENCE TIME AT ABOUT 650*C. OR HIGHER OF LESS THAN 0.5 SECOND, PREFERABLY LESS THAN ABOUT .1-.4 SECOND, A PRESSURE DROP OF ABOUT 3-5 P.S.I. PER TUBE AND A TUBE SKIN TEMPERATURE OF ABOUT 1000*-1200*C.

March 30, 1971 M. R. KITZEN ET AL 3,573,012

METHOD FOR STRAIGHT-THROUGH CRACKING OF HYDROCARBONS Filed Feb. 20, 1969 2 Sheets-Sheet 1 1/ I 1 1 W/ l I- II! .llI-Illl .i m m h. h m m II .L TIL INVENTORS. Maurice R. Kitzen Fraser M. Wall p l-FM ATTORNEYS.

March 30,1971 8 MR W N ETAL 3,573,012

METHOD FOR STRAIGHT-THROUGH CRACKING OF HYDROCARBONS Filed Feb. 20, 1969 2 Sheets-Sheet 2 L] n: g 5 8 8 8 8 8 0 TUBELENGTH Q3 8 8 8 8 8 Q O 01 0 'HHHLVHEIdINBi INVENTORS. Maurice R. Kitzen By Fraser M. Woll ATTORNEYS.

United States Patent O 3,573,012 METHOD FOR STRAIGHT-THROUGH CRACKING F HYDROCARBONS Maurice R. Kitzen, Elkins Park, and Fraser M. Wall,

Abington, Pa., assignors to Selas Corporation of America, Dresher, Pa.

Filed Feb. 20, 1969, Ser. No. 801,020 Int. Cl. C01b 2/14 US. Cl. 48-214 6 Claims ABSTRACT OF THE DISCLOSURE Hydrocarbon feed stocks are cracked by passing them in a straight linear path through elongated externally heated tubes of open cross-section, with a residence time at about 650 C. or higher of less than 0.5 second, preferably less than about .1-.4 second, a pressure drop of about 3-5 p.s.i. per tube, and a tube skin temperature of about 1000 -1200 C.

BRIEF SUMMARY OF THE INVENTION This invention relates to a cracking of hydrocarbon feed stocks and more particularly to a straight-through hydrocarbon cracking apparatus and process which produces a product having enhanced ethylene content.

BACKGROUND OF THE INVENTION (a) Field of the invention This invention lies in the field of hydrocarbon cracking, as exemplified by thee US. patent to Kitzen No. 3,353,920, granted Nov. 21, 1967.

(b) Description of the prior art Reference is made to the patent to Kitzen No. 3,353,920, granted Nov. 21, 1967. It is known that hydrocarbon cracking furnaces and processes have involved the use of conventional operating conditions for a sufficient time to reach substantially equilibrium conditions for recovery. The Kitzen patent taught that better results can be obtained on some cases by using especially severe conditions of temperature and heat input, and by then cutting off the reaction prior to the time that equilibrium is reached.

The Kitzen patent points out that various problems are overcome, when using a furnace having tubes arranged in a serpentine or coiled path, by using the so-called high severity process, in which the reactants are treated severely and then quenched before they have a chance to reach equilibrium conditions.

The use of coils in a high severity type furnace has always been considered highly advantageous because these coils present maximum exposure to heat exchange surface to the source of heat, and provide maximum dwell time for the reactants in the furnace because of the tortuous nature of the path through which the reactants flow.

SUMMARY OF THE INVENTION It has now been discovered that substantially enhanced ethylene contents can be obtained in the product, and that other advantages are achieved as well, by conducting the cracking operation in a furnace having a multiplicity of essentially straight elongated tubes which have completely free cross-sections, by passing the feed stock in a substantially straight linear path through one of these tubes, and establishing a high velocity of feed and withdrawal in order to provide a very low material residence time at a temperature above about 650 C. in the tubes of less than about 0.5 second, preferably only about .1.4 second, maintaining a total pressure drop in the tube from inlet to outlet of about 0.3-5 p.s.i., maintain- 3,573,012 Patented Mar. 30, 1971 ing a tube skin temperature of about 1-000l200 C., and suddenly quenching the reaction product as it emerges from the tube, long before the reactants have a chance to come to equilibrium.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE DRAWINGS The furnace shown in the drawing has a plurality of elongated straight tubes 10, extending vertically through the combustion zone 11 of the furnace 12. Heat is provided by a multiplicity of spaced apart burners B located on the inner surfaces of the furnace walls 13. Preferably the burners B are located on both opposed front and back walls of the furnace, in a manner to direct heat at opposite faces of each tube. The feed materials are introduced into the upper header 14 and are withdrawn through lower header 15 and pipe 16, to quench.

It has been found that, in a furnace provided with a multiplicity of parallel, straight tubes arranged in vertical relationship with one another, with burners in the opposed Walls of the furnace, it is possible to obtain substantially equal and uniform tube skin temperature profiles in all the tubes. This arrangement of the combustion components and the straight-through path of the material provides excellent duplication from tube to tube with a resulting profile which is considered to be unique in the art.

It is to be emphasized that the' tubes in accordance with this invention are straight-through, and are arranged to carry the reaction products once through, and only once, and that the tubes all have open cross-sections. They are not filled with catalysts or any other solid materials, such as the tubes that are referred to in the patent to Fleischer No. 3,062,167, assigned to the assignee hereof.

Preferably, the tubes are subjected to controlled heating so that the inner wall temperature of each tube is substantially constant fromone end of the tube to the other. This is accomplished by the spaced arrangement of the radiant heat burners B and by providing variable firing rate equipment for each burner, along with control means for controlling each firing rate, the latter being well known per se.

This invention is to be distinguished sharply from the use of serpentine tubes as in the aforementioned Kitzen Pat. No. 3,353,920. It has been found that serpentine tubes provide at least three times as much pressure drop, largely because of the bends, and that great operating improvements are surprisingly obtained in a once-through straight line system according to this invention.

The following table illustrates typical differences between a conventional high severity furnace in accordance with the Kitzen patent, and a straight-through furnace in accordance with this invention.

Outlet temperature 800 C.

Tube skin temperature 1050 C. Methane yield 16% Methane yield 14%.

EXAMPLE 1 A full series of tests was conducted, using a full range naphtha, at a predetermined throughput rate and at outlet temperatures varying between 740" C. and 840 C.

The results of these tests appear in the table which follows.

NAPHIHA CRACKING RUNS: PRODUCT ANALYSESPERCENT BY WEIGHT Run Number Product! Sum of 0 compounds and lighter 42. 35 48. 65 56. 25 58. 59. 1 61. 1

(1 1-1 1-3 3. 3 4.05 4. 4 4. 2 3. 9 3.6 0411 iso plus normal- 5. 2 4. 1 4. 6 3. 6 2.9 2. 3 C4Hscis/tra1'1S 1.6 1.65 1.45 1.2 1.0 0.8 C4H iso plus normal 2. 3 1. 4 1.05 0. 7 0.6 0. 4 Other C hydrocarbons 0. 05 0.05 0. 1 0. 1 0. 1 0. 1

Sum of C4 compounds 12. 45 11. 25 11. 6 9. 8 8. 5 7. 2

Benzene 3.0 3. 7 4. 75 7. 15 11. 0 10. 8 Toluene. 2. 4 2. 00 1. 85 2. 5 2. 4 2. 7 Xylene 1. 3 1. 1. 25 1. 1 0. 8 0. 6

BTX 6. 70 7. 05 8. 85 10. 75 15 10 14. 10

C 185-(BTX) 37. 7 29. 95 21. 2 16. 65 12. 10. 8

Oil 1. 00 1. 75 2. 9 4. 2 5. 0 6. 6

Grand total 100. 2 99. 65 99. 80 99. 70 100. 15 99. 80

Outlet temp., C 740 760 780 800 820 840 Naphtha iced density at 15 C 0. 700 0. 700 0. 700 0. 700 0. 700 0. 700

EXAMPLE 2 A series of naphtha tests was performed, using a furnace of the type appearing in the drawing, the straight naphtha and 1.6 metric tons per hour of steam. The crossover temperature and outlet temperature and the outlet pressure were as given below.

Run Number Material residence time-seconds above 650C 0.17 0.16 0.18 0.18 0.22 0.21 0.25 0.27 0.27 0.28 0.30 0.38 CrossovertemperatureC 534 573 566 596 595 632 615 625 636 Outlet tcmperatuIe,C.. 760 780 780 800 800 820 820 840 840 Outlet pressure (atm.) 0.72 0.80 0.83 0.80 0.80 0.90 0.83 0.85 0.85 1.10

tubes being about 35 feet long. Before the series of tests was started the furnace was decoked completely and several runs were obtained with the product outlet temperature at the exit from the tubes varying from 740 C. to 840 C. In all cases, the outlet header was located within the furnace area. Typical parameters of the feed stock are given below.

Run Number The number of burners B in operation was varied from 40 (during run 33) to 48 (during run 44), all burners operating at equal burner pressure of approximately 1 atmosphere and burner banks on both sides of the tubes being fired. At the bottom rows, near the outlet, no burners were used except during runs 42 and 43 when two burners were used for each run. When the tube skin temperature approached the maximum allowable temperature of about 1200 C., the two burners at the bottom rows were shut off during run 44. The average actual temperature profiles taken along the lengths of the tubes are shown in FIG. 2 of the drawings. In every case, the material residence time in the tubes, during Which the material was at a temperature of 650 C. or above, was less than 0.5 second and even in the range of 0.l0.4 second.

EXAMPLE 3 Another set of runs was conducted. Again before the series was started the furnace was decoked. The furnace outlet temperature was varied from 700 C. to 770 C., at design gas oil input and 67% dilution with steam.

Tube skin temperature as referred to herein is intended to indicate the temperature measured at the out- FEED SPECIFICATIONS Test; Number The characteristics of the gas oil feed stock were as follows;

Carbon, wt. percent 85. 5 Hydrogen, wt. percent.- 13. 5

5 5 2269541 3 .MOMMM4567913M A O 1222222222333 2 92 77234693O9 57 61 23 5790 2 U%fi22%22233%% m2 797581M088833 1 8 235 891 2 1%m2222223Mw ww 22 9 7282965 61 fiww o l34fi7 4 -amm 2 11222222233 3 58 29896273095 9 50 6 02457 0 2 imwzzazzwswmm 71 409271 052 41 7 235 8 1 2 lwm22222ad3mmo w Mus 0781482208077 0 7 23467 03 2 lww22222hd33fiwm 31 025 61647 1 40 5 1%m5780 2 1mn 4 22 229.3%Mo w 55 46 71359 92 7 41 7 24567 0 2 %m22222%3%%% 16 1816 422 5 40 5 134%7 1 2 2 mn a22222ww3mfimw H t n a w mt nn Qh 89 PE mm n w 1 mm wmm m am 0 emI 1 u SMB Run Number A summary of the main furnace conditions is given below.

The following table sets forth the results of these runs.

Material residence time, seconds above 650 C 0 Gas 011 feed (metric tons/hr.)

Steam (metric tons/1m)- Cross-over, C

Outlet pressure (atm.)

5 5 5 11 021 23 15 51%290 ofqmmmlo 0080B O331U0 heat to the tubes of the outlet portions than at the inlet portions. This tends to provide a substantially constant tube wall temperature from end to end.

The pressure drop from the inlet to the outlet of the 5 p.s.i. gauge for a typical tube that is about feet long. Tubes can be from 20-60 in length depending upon process requirements.

As stated, it is important to provide a quench at the outlet end of the tube. This is done by conventional quenching means well known in the art and not shown in the drawing.

It will be appreciated that the amount of the pressure drop varies from tube to tube and depends among other things upon the inside diameter of the tube. For a rather Grand total..-

N aphtha input; t./hr

In all of these runs, the material residence time as herein defined was less than 0.4 second. The results show that optimum conversion for by-product yields 1s at about ylene yield is still increasing, however, at the cost of 65 tube in accordance with this invention is less than about The reduced methane yield is advantageous because methane is much less desirable as a product than ethylene or propylene.

In accordance with the preferred embodiment of this tion, steam is introduced withthe hydrocarbon feed Steam dilution peroen 780? C. to 800 C. and that at higher temperatures ethpropylene and C production.

inven for the purpose of dilution. Approximately .3-l.0 pound of steam per pound of hydrocarbon feed are used. The steam helps to keep the coking low and keeps the partial pressure of hydrocarbon feed low.

small size commercial tube having an inside diameter of 3.21 inch, a pressure drop of 0.5 p.s.i. has been measured. However, the pressure drop can go up to as high as about 2 p.s.i. during the operation of the apparatus, particularly if some coking is experienced after an extended period of operation.

It will also be appreciated that the hydrocarbon partial pressure changes as the reaction proceeds and also varies depending upon the amount of dilution steam that is incorporated into the feed. An increased dilution proportion produces more olefins and less methane in the product.

The feed stock in accordance with this invention can be essentially any feed stock which is known to be useful for the production of olefins such as ethylene and the like, and includes all the feed stocks referred to in the aforesaid Kitzen Pat. No. 3,353,920. Typical of such feed stocks are gas oils, naphtha, liquid petroleum gas feeds, ethane, protane, butane, etc.

Although this invention has been described with reference to a specific form thereof, it will be appreciated that other variations may be rnade without departing from the spirit and scope of the invention.

The following is claimed:

1. In a method of cracking a hydrocarbon feed stock, the steps which comprise feeding said stock in a substantially straight linear path through an elongated tube, maintaining said linear path free of any obstruction, establishing a velocity of feed and a velocity of withdrawal of product, to provide a material residence time in the tube at a temperature above about 650 C. of less than about 0.5 second, maintaining a total pressure drop in the tube from inlet to outlet of about 0.3-5 p.s.i., maintaining a tube skin temperature of about 1000-1200 C. and suddenly quenching. the reaction product as it emerges from the tube.

2. The method defined in claim 1, wherein the feed is diluted with about .3 to 1.0 pound of steam per pound of hydrocarbon.

3. The method defined in claim 1, wherein the tube skin temperature is about 1050 C.

4. The method defined in claim 1, wherein the product temperature as it leaves the tube is maintained at about References Cited UNITED STATES PATENTS 3,407,789 10/1968 Hallee et al. 4-8--214X MORRIS O. WOLK, Primary Examiner R. E. SERWIN, Assistant Examiner US. Cl. X.R. 

